Diving into Mercury's Magnetosphere

During its third flyby of Mercury, the space probe BepiColombo discovered unknown plasma structures and a possible ring current.

During its third flyby of Mercury in June 2023, the European-Japanese space probe BepiColombo succeeded in taking new measurements in the planet’s magnetosphere. BepiColombo passed through Mercury's magnetic shield for around 30 minutes, allowing the scientific instruments on board to determine the distribution and properties of charged and uncharged particles there. A group of researchers, including scientists from the Max Planck Institute for Solar System Research in Germany, report on the results in the journal Communications Physics. The researchers, led by the Paris Observatory, discovered a previously unknown layer of turbulent plasma at the edge of the magnetosphere as well as evidence of a ring-shaped current of energetic ions around the planet. Until the space probe enters an orbit around Mercury in 2026, its flight route will pass by the planet a total of six times. The fourth maneuver of this kind took place just under a month ago. The data obtained during this maneuver is currently being analyzed.

Mercury's magnetic environment is unique: the strength of the magnetic field generated in the planet’s interior is only a tenth of the value on Earth; at the same time, as the planet closest to the Sun, Mercury is exposed to a particularly intense bombardment of solar wind particles. While the Earth's magnetic field shields a large part of these particles, the situation around Mercury is much more complex. “At Mercury, the solar wind particles can penetrate the magnetosphere particularly easily - and even reach the planet’s surface,” explains MPS scientist Dr. Markus Fränz, co-author of the current study. Mercury's plasma environment is therefore largely determined by the interaction between the surface, particles and magnetic field.

In the current publication, the researchers report on low-energy oxygen, sodium and potassium ions, for example, which the space probe was able to measure in the magnetosphere on the planet’s night side. They likely originate from its surface, from where they diffuse through the heating of the day side of the planet or are ejected by solar wind particles or tiny mini-meteorites.

Foretaste with limited field of view

In the past, earlier Mercury travelers such as the American space probes Mariner 10 and MESSENGER have been able to sketch the basic structure of Mercury's magnetosphere. After BepiColombo’s arrival at Mercury, the Mercury Plasma Particle Experiment (MPPE) instrument package, among others, will enable more detailed and comprehensive measurements. The MPS is involved in the mass spectrometer MSA (Mass Spectrum Analyzer), a sub-instrument of MPPE. The total of six Mercury flybys offer a first taste of the measurements that can then be expected.

However, the field of view of the MPPE instruments is significantly restricted in the flight configuration. BepiColombo consists of two probes, ESA’s Mercury Planetary Orbiter (MPO) from ESA and Jaxa’s Mercury Magnetospheric Orbiter (MMO), which are traveling “piggybacked” on top of each other, so to speak. Their paths will only separate once they have entered into orbits around the planet. Until then, the solar shield of MMO and the transfer module blocks part of the view. 

Nevertheless, the measurements taken on June 19 of last year provide an impressive picture of the plasma environment. Coming from the night side, the trajectory took BepiColombo almost in the equatorial plane to within 235 kilometers of the planet's surface, crossing all areas of the magnetosphere in around 30 minutes.

“We saw expected structures like the ‘shock’ boundary between the free-flowing solar wind and the magnetosphere, and we also passed through the ‘horns’ flanking the plasma sheet, a region of hotter, denser electrically charged gas that streams out like a tail in the direction away from the Sun. But we also had some surprises”, says first author Dr. Lina Hadid from the Paris Observatory, summarizing the results.

Boundary layer and ring current

One of the surprises is a kind of boundary layer that BepiColombo discovered at low latitudes at the edge of the magnetosphere. The particles there apparently exhibit a significantly wider range of energies than has ever been measured on Mercury before. In addition, there are repeated bursts in the ion density. The team suspects that an as yet unknown process is at work here, which supplies the plasma with “fresh” particles.

Closer to the planet, the space probe came across a region of particularly high-energy ions. The researchers believe this to be evidence of a so-called ring current. This refers to a current of charged particles trapped in the magnetic field that flows around the planet in the equatorial plane. The terrestrial ring current runs at a height of around 20,000 to 60,000 kilometers above the Earth's surface, where it generates a magnetic field that opposes the Earth's actual magnetic field. “The new measurements indicate that Mercury's ring current is located much closer to the planet. As the mission progresses, we hope to understand how this is possible,” says MPS scientist and co-author Dr. Norbert Krupp.

Following the fourth Mercury flyby about a month ago, the next ones are already imminent: the next opportunities to collect measurement data from the planet's magnetosphere will be on December 1 of this year and January 8, 2025.